1. Field of the Invention
The present invention relates to a hydrogen occlusion alloy for electrical cells which is used as a negative electrode material in closed type nickel hydride secondary cells, more particularly to a hydrogen occlusion alloy for electrical cells which contributes to reduction of the gas pressure in the cells upon overcharge thereof and prevention of self discharge of the cells in the open state.
2. Prior Art
Development of nickel hydride secondary cells is being realized as electric power sources for various portable electronic instruments.
The nickel hydride secondary cells operate using hydrogen as negative electrode active material and comprise a positive electrode comprising an active material, Ni(OH).sub.2 powder, supported on a collector and a negative electrode comprising a hydrogen occlusion alloy powder reversibly occluding and releasing hydrogen electrochemically and supported on a collector, wherein the positive and negative electrodes are contained together with an alkaline electrolyte and a separator interposed therebetween in a packaging vessel which also functions as a negative electrode terminal, the packaging vessel being closed with a cover or lid which also functions as a positive electrode terminal.
In the closed nickel hydride secondary cells, the charging and discharging reaction in the negative electrode is represented by the following equation: EQU M+aH.sub.2 O+ae.sup.- MHa+aOH.sup.- ( 1)
wherein M denotes a hydrogen occlusion alloy. Thus, upon charging the hydrogen occlusion alloy occludes atomic hydrogen produced from the water constituting the alkaline electrolyte by electrochemical reduction, while upon discharging the occluded hydrogen is electrochemically released by oxidation to water.
Various hydrogen occlusion alloys having such a function are known and those alloys represented by the following formula:
AB.sub.5
wherein A denotes a misch metal and B denotes such an element as Ni, Co, Mn or Al are widely used.
On the other hand, the charging and discharging reaction in said positive electrode is represented by the following equation: EQU Ni(OH).sub.2 +OH.sup.- NiOOH+H.sub.2 O+e.sup.- ( 2)
Thus, upon charging Ni(OH).sub.2 is electrochemically oxidized to NiOOH (nickel oxyhydroxide) and upon discharging the reverse reaction takes place.
In the closed nickel hydride secondary cells, the capacity of the negative electrode should be designed to be larger than that of the positive electrode for the following reasons.
Upon overcharging, oxygen is generated from the positive electrode prior to the generation of hydrogen from the negative electrode. And the generated oxygen should be absorbed by the negative electrode based on the reaction represented by the following equation: EQU O.sub.2+2 H.sub.2 O+4e.sup.- .fwdarw.4OH.sup.- ( 3)
or the following equation: ##EQU1## whereby the gas pressure in the cell should be prevented from excessively raising.
Recently, as portable electronic instruments have rapidly been popularized, there is a strong need for further higher capacities of nickel hydride secondary cells as electric power sources.
As measures to meet such a need, the amount of Ni(OH).sub.2 or hydrogen occlusion alloy to be supported on the collector may be as large as possible so as to increase the capacity of the positive and/or negative electrode, or an active material having a high capacity per unit volume may be used.
However, such measures have already been done approximately to the maximum extent and it would be difficult to realize further higher capacities by these measures.
As other measures the gas pressure in the cell during overcharging may be reduced, since if the gas pressure in the cell is low the excess capacity of the negative electrode required to prevent the generation of hydrogen during overcharging can be reduced.
In such cases, hydrogen occlusion alloys as mentioned below are preferred as those constituting the negative electrode.
First of all, the hydrogen occlusion alloy should have a high activity and a low overpotential upon charging for the following reasons. Since most of gaseous materials in the cell upon overcharging is hydrogen gas generated from the negative electrode the use of a negative electrode comprising a hydrogen occlusion alloy with a low overpotential could prevent the leakage of hydrogen upon charging resulting in prevention of increase of the gas pressure.
To reduce the overpotential of a hydrogen occlusion alloy, it is preferred that the effective surface area for the charging reaction is large.
In general, such an effective surface area can be realized by pulverizing the powder of a hydrogen occlusion alloy used. However, it is not preferable to use pulverized powder from the first step in the production of negative electrodes The pulverized powder may be inactivated by oxidation of the surface thereof may not necessarily have an increased effective surface area for the charging reaction.
Therefore, when a hydrogen occlusion alloy having an increased surface area for the charging reaction is used, it is preferred that the hydrogen occlusion alloy is pulverized in the charging and discharging process of the cell and a new active surface is generated upon the pulverization.
A hydrogen occlusion alloy will have a property of being pulverized by the occlusion and release of hydrogen. Generally, however, pulverization will proceed fully only when hydrogen is occluded to the maximum extent of the hydrogen occlusion capacity of the hydrogen occlusion alloy followed by release thereof
However, since the capacity of the negative electrode is larger than that of the positive electrode, a hydrogen occlusion alloy constituting the negative electrode of a closed nickel hydride secondary cell, can not occlude hydrogen to the maximum extent of the hydrogen occlusion capacity. Therefor the amount of hydrogen occluded during the charging of the cell will be at a low level far below the maximum value.
Thus, the pulverization of the hydrogen occlusion alloy will not proceed fully in the charging and discharging process and the newly formed active effective surface area will not be satisfactory.
Japanese Patent Application Laying Open No 2-277737 proposes a hydrogen occlusion alloy having the following composition: EQU ANiaCobMnc (5)
wherein A is a rare earth element including La, 2.5.ltoreq.a.ltoreq.3.5, 0.4.ltoreq.b.ltoreq.1.5, 0.2.ltoreq.c.ltoreq.1.0, and a+b+c is 3.85 to 4.78, inclusive; or EQU ANiaCobMncXd (6)
wherein A is a rare earth element including La, X is at least one element selected from the group consisting of Fe, Mo, W, B, Al, Si and Sn, 2.5.ltoreq.a.ltoreq.3.5, 0.4.ltoreq.b.ltoreq.1.5 0.2.ltoreq.c.ltoreq.1.0, 0&lt;d.ltoreq.0.3, and a+b+c+d is 3.85 to 4.78, inclusive.
These hydrogen occlusion alloys belong to the aforementioned AB.sub.5 type wherein the compositional ratio of the element A to the element B is non-stoichiometric. They have a property of being pulverized even when they occlude only a small amount of hydrogen.
Indeed, the hydrogen occlusion alloys represented by the formulae (5) and (6) will be pulverized even by occlusion of a small amount of hydrogen.
It would be therefore expected that the active surface area for the charging reaction could be increased and the increase of the gas pressure in the cell upon overcharging could be prevented. However, the raise of the gas pressure is not so significantly prevented in the actual cases.
Furthermore, when a cell is allowed to stand in the open circuit state, self discharge of the cell strongly proceeds causing a problem of reducing the discharge properties.